A couple of update reports.
Sept. 19
This week's update. September 19
I used an oscilloscope to check some things. I checked with the
engine running with A-B jumpered, and then pulled the jumper out to see
if there was any difference. Here's what I checked.
Distributor Reference (pin D8 on the PCM). This is a crank position
signal, and should be a regular square wave with 8 pulses for every
crank rotation. This signal is supplied to the PCM by the distributor.
It looked OK for both cases, considering that the frequency wasn't
steady as the engine sputtered with the jumper out.
Electronic Spark Timing (pin C8 on the PCM). This is supposed to be like
the crank signal, but advanced or retarded as required by the engine.
This signal is supplied to the distributor by the PCM. This also looked
normal. Of course, I couldn't tell if it was advanced or retarded.
Cam Position Sensor (pin C5 on the PCM). According to the manual, this
is supposed to be one pulse for each time the intake valve for cylinder
one is about to open. This signal is supplied to the PCM by the
distributor. This also looked normal, but there's no way I could tell if
it was in the right place.
The belt was thrown when I pulled the jumper in this test, so that's all
I did for now.
There is another line, Bypass (C7 on the PCM), controls whether the ICM
uses the EST signal or the signal directly from the pickup coil to
trigger the spark coil. I need to measure that line with the jumper in
and out to see if that is the mechanism for using fixed park with A-B
jumpered. If so, that would mean that the PCM is doing nothing at all
with A-B jumpered, as far as ignition goes.
But the belt was thrown when I pulled the jumper in the Cam Position
test, so that's all I did for now.
Sept. 21
Today I found that the Bypass signal is always high, which means that the spark is always controlled by the EST siignal, regardless of whether A-B is jumpered or not. So the PCM must send the crank signal directly to the EST output when the jumper is in place, and controls it when the jumper is out. So the timing must be thrown way off when the jumper is out. The only thing that I can think of that would cause that is the cam signal must be way off.
BTW, if the Bypass line is disconnected, the engine really does run directly off the pickup coil, and the ICM shorts the EST line to ground. It actually runs quite smoothly this way, but at a fixed advance. The Bypass line must be a kind of limp home thing in case the PCM spark control completely fails. The Bypass line must be at 5 volts in order for the ICM to use the EST signal.
Also, I figured out why it is throwing the belt. Compared to a picture from Pennock's, the tensioner is in the wrong place. I'd have to make a new home-made bracket to fix it, and get a new belt, because the existing one would then be too long.
